This invention relates generally to orthopaedic surgery and, more particularly, to a proximal femoral prosthesis facilitating an enhanced range of motion.
In total hip arthroplasty, the defective head and neck of the proximal femur are removed and replaced with a prosthetic element. Although extramedullary units are available, intramedullary prostheses are more commonly employed, which feature an elongated stem adapted for insertion and fixation within the femoral canal.
FIG. 1 is a generalized representation of a prior-art proximal femoral endoprosthesis. A head portion 102 having an outer surface 103 which is at least partially hemispherical is joined to a stem 106 through a neck portion 104. Such interconnections may be permanent and integral, or modular connections may be used in conjunction with tapered metal joints, for example.
The stem 106 defines a first axis 108 which is aligned more or less to the longitudinal axis of the femur, depending upon the style of the particular implant. The neck 104 defines a second axis 110 which intersects with the first axis 108 at a neck/shaft angle which may be varied in accordance with the physiology of the recipient or the desires of a given manufacturer. A typical neck/shaft angle xcex1 is on the order of 135xc2x0. The offset, or distance from the head portion to the axis of the stem, may also varied to achieve a desired result. A number of other variations exist, including cemented versus cementless interfaces, curved versus straight stem profiles, differently sized balls, and so forth.
In all existing configurations, the neck is straight or, in some cases, curved upwardly (or proximally) away from a plane transverse to the axis of the stem. That is to say, a centroid drawn from a central region 112 of the head 102 to a point of intersection 111 with the stem axis 108 is straight or occasionally curved to create a convex neck surface in existing designs. Such a configuration has several shortcomings. For one, as manufacturers decrease the neck-shaft angle xcex1 to improve offset and abductor tension, patients lose movement in flexion secondary to impingement of the neck on the acetabular component.
This invention resides in proximal femoral prostheses which minimize impingement, thereby affording an enhanced range of motion as compared to existing devices. Broadly, the central portion of the neck of the inventive prosthesis is oriented downwardly relative to a straight line drawn between the ball portion and the point of interconnection to the exposed portion of the stem. Such a configuration reduces impingement in flexion/internal rotation and extension/external rotation, assuming an appropriately placed acetabular component. In the preferred embodiment, the neck is curved between the head and the neck, though, in alternative embodiments, the neck may be provided in straight and/or modular segments. The invention is compatible with neck-shaft angles, offsets, head sizes, and other dimensions commonly designated with respect to available implants. The neck may also be curved in the transverse plane adding increased anteversion or retroversion to the neck-shaft relationship.